1. Technical Field
The present invention relates generally to an electromagnetic coil suitable for use under application of high voltage, and more particularly to an ignition coil which develops high voltage to produce a spark as for ignition purposes in an internal combustion engine.
2. Background of Related Art
Japanese Patent Second Publication No. 2-18572 and Japanese Patent First Publication Nos. 2-106910 and 60-107813 teach conventional electromagnetic coils. These electromagnetic coils are made up of a plurality of slant winding layers oriented at a given angle to the length of a spool so that each of the slant winding layers presents a circular cone. In the following discussion, this type of electromagnetic coil will be referred to as a slant winding electromagnetic coil. The slant winding electromagnetic coils may be distinguished in the shape of winding layers from typical electromagnetic coils made up of cylindrical winding layers each extending in a lengthwise direction of a bobbin.
In such a slant winding electromagnetic coil, since each winding layer, as discussed above, extends radially so as to form a circular cone, the number of turns thereof is smaller than that of each of the cylindrical winding layers. This means that it is possible to decrease the number of turns of adjacent two of the winding layers to decrease a potential difference between the adjacent winding layers, thereby avoiding the dielectric breakdown for realizing an electromagnetic coil suitable for use under application of high voltage.
Such an electromagnetic coil is, as discussed in the above publications, suitable for use in an ignition coil for internal combustion engines. Particularly, this type of electromagnetic coil may be employed as a secondary winding for developing high voltage in combination with a primary winding.
The results of tests performed by the inventors of this application, however, showed that it was very difficult to arrange slant winding layers on a spool perfectly in an industrial manufacturing process, especially because an automatic winding machine which makes coils at high speeds is usually used in the industrial manufacturing process, and it is necessary to use thin wire for achieving the compact and lightweight structure of a coil.
The slant winding requires the formation of a cone-shaped winding using a leading portion of wire to define a reference surface for arranging slant winding layers in a lengthwise direction of a spool. In order to form the cone-shaped winding easily, it is useful to make an irregular winding of a triangle shape in cross section using a leading portion of wire, but a drawback is encountered in that it is difficult to develop a potential difference across each turn of the irregular winding at a constant level.
In the slant winding process, winding layers made of a trailing portion of wire may be shifted or crumbled.
The turns of wire may be disordered at the end of winding due to a variation in length of a spool, a variation in tensile force acting on the wire during winding, or undesirable insertion of a portion of the wire into a groove formed in a flange provided at an end of the spool for withdrawing an end of the wire.
When the above discussed irregular winding or irregularity of the winding caused by the disorder of the turns is included in the slant winding layers, it may cause some of the turns creating high voltages to be arranged adjacent to each other. It thus becomes difficult to estimate and manage the potential difference between the turns so that it is difficult to achieve high insulation expected in the slant winding electromagnetic coils.